Chapter 15. Troubleshooting BGP

This chapter covers the following troubleshooting topics:

• Troubleshooting BGP neighbor relationships

• Troubleshooting BGP route advertisement/origination and receiving

• Troubleshooting a BGP route not installing in routing table

• Troubleshooting BGP when route reflectors are used

• Troubleshooting outbound traffic flow issues because of BGP policies

• Troubleshooting load-balancing scenarios in small BGP networks

• Troubleshooting inbound traffic flow issues because of BGP policies

• Troubleshooting BGP best- path calculation issues

• Troubleshooting BGP filtering

This chapter discusses common and efficient real-life techniques to solve problems seen in running BGP networks. Cisco's implementation of BGP is fairly easy to configure, and robustness of Cisco IOS Software offers BGP operators great flexibility to use BGP to attain the most benefit. However, problems are unavoidable and things go wrong in real networks every day. This chapter offers a simple methodology to tackle problems in net-works running BGP.

To troubleshoot BGP- related problems, operators must start from basics. Most of BGP problems are similar to Open System Interconnection (OSI) model problems. For example, BGP neighbor relationship issues should be tackled by looking first at the nature of the neighbor relationship (IBGP or EBGP), followed by the physical connection between two BGP neighbors (OSI Layer 1); then encapsulation issues between neighbors (OSI Layer 2), IP connectivity (OSI Layer 3), and finally TCP connectivity (OSI Layer 4) should be considered . This troubleshooting method offers consistent and accurate resolution to the problem.

Cisco IOS Software debugs should not be run as the first troubleshooting tool. CPU- intensive debugs with a huge amount of data sometimes might not offer any help in troubleshooting a problem; instead, they can cause severe instability to the router.

It is impossible to discuss all BGP-related problems, but this chapter covers most of the problems seen in our real-life experience gained from working with networks running BGP on Cisco devices.

The flowcharts that follow document how to address common problems with BGP with the methodology used in this chapter.